[Interhemispheric differences of the middle latency auditory evoked magnetic fields revealed by a whole head magnetoencephalography system]

It is known that the auditory evoked N100m response appears faster in the contralateral hemisphere than in the ipsilateral hemisphere to the stimulated ear. For the middle latency response named P50m, however, there have been only few studies with controversial findings. In the present study, auditory evoked responses to the unilateral tone stimuli were measured in 36 right-handed normal subjects, using a helmet-shaped whole head magnetoencephalography system. In a total of 72 hemispheres of 36 subjects, P50m dipolar patterns were observed statistically (p > 0.02) frequently in the contralateral hemispheres (63.9%) compared to the ipsilateral hemispheres (43.0%). N100m dipolar patterns were observed over both left and right hemispheres either by left or right ear stimuli. Peak latency of the P50m was statistically (p < 0.02) shorter in the contralateral hemisphere (45.0 +/- 8.5ms, mean +/- s.d.) compared to the ipsilateral hemisphere (54.4 +/- 11.5ms). Peak latency of the N100m was also statistically (p < 0.001) shorter in the contralateral hemisphere. Moreover, when compared between contralateral responses, the left-ear stimulated right-hemispheric response (84.6 +/- 8.4) was statistically (p < 0.05) faster than the right-ear stimulated left-hemispheric response (89.3 +/- 10.2ms). These results indicate that contralateral auditory cortex dominates the ipsilateral cortex at the middle as well as the long latency components. Moreover, at least for pure-tone stimuli, right hemispheric dominance was indicated for the N100m responses. The whole-head MEG system is suitable to analyze small but significant interhemispheric difference of auditory function.